As known, satellite positioning systems comprise a constellation of satellites being arranged on average altitude orbits (of the order of 25,000 km) around the Earth. Such satellites and the orbits thereof are generally referred to in the art as “MEO satellites” and “MEO orbits”, respectively (for “Medium Earth Orbit”). The MEO satellites are evenly distributed in several orbit planes, so that, in any point of the Earth, a user is able to see several MEO satellites, that is being directly connected to them (at least three, but four if the user wants to know his altitude) and deduce therefrom his own earth coordinates. Each MEO satellite carries a high stability and accuracy atomic clock, as well as an electronic equipment sending to users positioning signals comprising the corrected time (brought back to a common reference) and the ephemerides of the satellite. From such positioning signals received from several MEO satellites, the user determines his distance to the various visible satellites and therefore deduces therefrom his position in earth coordinates. The present method, system and device aims at checking the integrity of such positioning signals.
It is known that usual solutions uses earth stations being dedicated therefor. Such stationary stations permanently compare the information received from satellites (via positioning signals) their accurately known effective position, so as to optionally detect a cohesion matching error in the received signals. Possible local deviations are transmitted to one or more control centres processing the information and identifying the satellite(s) transmitting erroneous positioning signals. Such quality information from the received positioning signals are transmitted to users later on, so that they ignore the erroneous signals in their positioning calculation.
The error causes in the positioning, signals being received can be ranked in two large categories, that is:
common errors due to satellites. Such errors are mainly due to the drift of the on-board clock giving the time reference, to the drifts of orbit parameters of the satellites or to processing anomalies on-board the satellites; and
local errors due to the propagation of signals through the atmosphere and/or to local multiple path phenomena. Such errors vary with time, simultaneously affect only a limited number of users, and are not of the direct responsibility of the operator who has no means for correcting them, as they depend on natural physical phenomena.
As the satellite positioning systems all use the measurement of differences of arrival time for the positioning signals emitted by satellites for calculating the earth position, and as both above mentioned error categories have an identical effect, that is, a time error of the signals received by the user, it is easy to confuse a local propagation error with a time error of the signal sent by satellites. In order to try and suppress such an ambiguity, dedicated processing centres collect information coming from numerous stations on the ground, and, through an essentially statistic calculation, separate the common error part from the local error part. Indeed, only the common error part, that is that coming from one or more satellites, is worth being transmitted to all users. Such a complex processing is time consuming, as it involves having to wait for enough information in order to distinguish without any ambiguity that part being local to one or more stations (such as an ionospheric effect or a geometry of the constellation leading to multiple paths) from the part being common to all stations and the cause of which can thus only be located on-board one or more satellites.
Until the positive identification of the erroneous satellite(s) is completed, all users will make an error in their positioning calculation, with potentially severe consequences, in particular, for mobile devices. It is easily understood that reducing the identification time of erroneous (satellite) positioning signals is a priority for any operator of such a positioning system, with a view to ensure the quality of his system to users.